The invention relates to a brake control system for motor vehicles and, in particular, to a hydraulic control system. The invention also applies to hybrid braking systems such as those provided in hybrid vehicles (vehicles propelled electrically and propelled using internal combustion engines) comprising a hydraulically operating braking system and an electric braking system using the electric propulsion motor or motors as electric generators.
In the prior art of motor vehicle hydraulic braking systems, a brake booster essentially comprises a space comprising two chambers (the front chamber or vacuum chamber and the rear chamber or working chamber) which are separated by a moving membrane secured to a piston. A control rod can move towards the front of the vehicle when the driver of the vehicle operates the brake pedal. This movement of the control rod is passed on to a plunger which actuates boost means and the booster piston. In general, these boost means comprise a three-way valve, actuation of which is able to interrupt communication between the vacuum chamber and the working chamber of the booster and to connect the latter chamber to the ambient atmosphere. Given that the vacuum chamber is normally under vacuum, and because of the pressure difference between the two chambers, a boost force is applied to the piston separating the two chambers. The piston therefore moves forwards, acting on a push rod that serves to actuate the master cylinder of the braking circuit.
The control rod which is actuated by the brake pedal is in contact with the piston of the booster, which is in contact with the push rod which acts on the piston of the master cylinder. The various parts that couple the brake pedal to the master cylinder piston are therefore in contact with one another. The driver therefore feels the reaction of the braking circuit through the brake pedal.
However, if a device on the vehicle displaces some brake fluid from some point in the braking circuit towards the master cylinder then there will be a reaction on the brake pedal and this reaction will be felt by the driver. For example, under braking that would have a tendency to lock up the wheels of the vehicle, the antilock braking system (ABS) has the task of reducing the braking effort and in order to do so of extracting brake fluid from the wheel cylinders and injecting it into the master cylinder. Alternatively, in electronic stability programs (ESP) that provide dynamic course control, a hydraulic unit is capable of acting on one or more braking circuits independently of the brake command and this action is also felt at the brake pedal and, if the driver is braking when the ESP cuts in, he will experience a variation in the feel of the brakes which will not necessarily correspond to the feel to which he is accustomed.
It should also be pointed out that because the effect of the hydraulic unit injecting brake fluid into the braking circuit is to move back the pistons of the master cylinder, it also has the effect of causing the brake pedal to move back. If this occurs at the very moment that the driver is exerting a relatively high braking force, then the backward movement of the brake pedal is absorbed by the driver's ankle and this may, at the very least, prove to be unpleasant for the driver. In addition, a backward movement of the master cylinder pistons may even result in bodily injury (for example may break the driver's ankle for example) in an accident, particularly a frontal impact, that occurs while the driver is applying a strong pressure to the brake pedal.
It may therefore prove beneficial for the onward transmission of all these effects that are brought about in the braking circuits and/or in the master cylinder, towards the brake pedal, to be lessened, (if not eliminated or absorbed).
One way to resolve these disadvantages is to provide electrical control of the control system and to provide a system which applies to the brake pedal mechanical commands that simulate the feel of the brakes that the driver is accustomed to feeling with a conventional braking system under the same braking conditions. In the remainder of the description, this device will be termed a “brake feel simulator”. In a system such as this, the actual braking devices are therefore disconnected from the brake feel simulator which in response to the braking applies effects to the brake pedal.
However, such systems are expensive by comparison with conventional hydraulic braking systems. Introducing electrical controls and connections generally presents problems with reliability. These systems are therefore even more expensive if they are subjected to the same reliability and safety requirements as conventional hydraulic systems.
Furthermore, so-called hybrid vehicles which have both an electric propulsion motor powered by batteries for propelling the vehicle and an internal combustion engine (running on gasoline, diesel oil, gas or any other fuel) are generally equipped with electric brakes in which braking is obtained by inductive braking with the recuperation of energy from the electric propulsion motor of the vehicle. The electric propulsion motor then acts as an electric generator, the electrical energy recuperated being used to recharge the batteries, something which is advantageous in the use of the vehicle.
In these systems, it is also possible to vary the electric braking torque. Under braking, the maximum amount of electric braking is not always therefore applied. This may for example be the case when use is being made of a radar providing information on the road condition, or when operating the brake pedal somewhat quickly (for example when braking hard and then releasing the brake pedal).
Provision may also be made for an opposite current to be transmitted to the electric motor either for safety reasons or for reasons associated with the feel of the brake pedal.
However, electric braking is not entirely satisfactory. FIG. 1 schematically in continuous line shows a braking curve (braking torque as a function of time) of an electric braking system such as this alone. This same graph also shows, in dotted line, the speed of the vehicle. In a first braking region A, the braking increases progressively until it reaches full effectiveness in a second braking region B, then, as the vehicle speed becomes low, in a third region C, the braking torque decreases and becomes practically non-existent in a fourth region D. A braking system as this is imperfect because in region A braking does not become fully effectively quickly and because in regions C and D the braking effect diminishes when the vehicle is at low speed. Operation such as this is depicted in FIG. 1 by the curve in dotted line.
To remedy this disadvantage of recuperative electric braking, it is necessary for a hydraulic braking system to provide supplementary braking in regions A, C and D.
FIG. 2 shows curves of the operation of the electric braking system and of the hydraulic braking system.
Curve C1 is the recuperative electric braking curve which corresponds to that of FIG. 1. If a vehicle deceleration of 0.7 g for example is desired, and the electric braking system is capable of achieving a maximum deceleration of 0.3 g, then the braking curve applied by the hydraulic braking system needs to be schematically that represented by curve C2. In region A, the hydraulic braking torque will achieve a deceleration of 0.7 g as quickly as possible and will then decrease until the electric braking torque reaches its maximum value and the equivalent deceleration due to this electric braking torque is 0.3 g. In this way, at every moment the sum of the torques supplied by the electric and hydraulic braking systems provides a deceleration of 0.7 g. In region B, the hydraulic braking system supplements the electric braking system in order also to obtain a deceleration of 0.7 g. Thereafter, in region C, the braking torque of the hydraulic system is increased in order once again to provide supplementary braking to the electric braking system and, in region D1, to remedy the fact that electric braking torque is practically nonexistent.
Furthermore, disregarding the response times of the recuperative electric braking system, this system is not able always to respond in the same way because the load on the recuperative braking circuit may vary. This is particularly true when the recuperative circuit essentially includes the vehicle batteries. In such a case, the load can vary according to the state of the batteries.
In such systems, a control circuit, for example a computer, has to manage the operation of the braking systems. This computer is called into operation for each braking action. It has been found that electrically controlled systems are vulnerable whereas hydraulically controlled systems using brake boosters are proven and remain less expensive than electrically controlled systems.